The objectives of the proposed research are: (1) To design, construct and test a system for conveniently, accurately, and simultaneously irradiating cell cultures with microwaves and x-radiation at pre-selected and controlled elevated temperatures between 37.5 degrees C and 45 degrees C. (2) Using this system, to subject cells to range of microwave power levels at carefully controlled temperatures; the microwave power levels will range from a few milliwatts/cm2 to powers high enough to produce massive thermal cell killing in exposure times less than one minute, if the excess energy were not carried off by an efficient cooling system. (3) Using cell survival and cell membrane damage as primary cirteria, to determine whether or not: (a) The effects of microwave hyperthermia are different from water bath heating, and; (b) Hyperthermia produced by pulsed microwave sources yields significantly different effects from those caused by continuous wave microwave sources operating at the same average power levels. (4) Using cell survival and cell membrane damage as primary criteria, to determine whether or not there are significant differences between the effects of: (a) Simultaneous x-irradiation and water bath heating, and; (b) Simultaneous x-irradiation and microwave-induced hyperthermia. It is essential to determine the effects of strong electromagnetic fields (high microwave power) at elevated temperatures, since short exposures at high instantaneous power levels are being used to develop shaped thermal fields for hyperthermal treatment of cancer patients.